The present invention relates to novel devices and methods to optimize the production of hydrocarbons from subterranean reservoirs.
One of the first steps in oil and gas production is drilling a wellbore into the hydrocarbon-bearing formation. Next, a casing (liner), generally steel, is inserted into the wellbore. Once the casing is inserted into the wellbore, it is then cemented in place, by pumping cement into the gap between casing and borehole (annulus). The reasons for doing this are many, but essentially, a liner helps ensure the integrity of the wellbore, i.e., so that it does not collapse; another reason for the wellbore liner is to isolate different geologic zones, e.g., an oil-bearing zone from an (undesirable water-bearing zone). By placing a liner in the wellbore and cementing the liner to the wellbore, then selectively placing holes in the liner, one can effectively isolate certain portions of the subsurface, for instance to avoid the co-production of water along with oil.
That process of selectively placing holes in the liner and cement so that oil and gas can flow from the formation into the wellbore and eventually to the surface is generally known as xe2x80x9cperforating.xe2x80x9d One common way to do this is to lower a perforating gun into the wellbore using a wireline, slickline, or coiled tubing to the desired depth, then detonate a shaped charge mounted on the main body of the gun. The shaped charge creates a hole in the adjacent wellbore liner and formation behind the liner. This hole is known as a xe2x80x9cperforationxe2x80x9d. U.S. Pat. No. 5,816,343, assigned to Schlumberger Technology Corporation, incorporated by reference in its entirety, discusses prior art perforating systems (e.g., col. 1., 1. 17).
In the U.S. patent application Ser. No. 09/321,040, filed May 27 1999, and assigned to Schlumberger Technology Corporation, a method and apparatus is disclosed for minimizing the sand production from a wellbore by generating essentially elliptically shaped perforations. It was found that a particular shape and orientation of the perforation minimizes the destabilization of sand formations, hence also minimizes sand production. As a particular example, elliptically shaped perforations having the major axis of the ellipses aligned in the direction of maximum compressive stress, improve the stability of the formation in the region near the wellbore, hence can minimizing sand intrusion.
One of the co-inventors of the above-identified patent application now found that essentially elliptically shaped perforations have a potentially beneficial, i.e., a maximizing effect on the production of hydrocarbons from a producing well, when properly oriented with respect to the surrounding formation.
In accordance with the present invention, there is provided a method and apparatus for increasing hydrocarbon production from a production well by determining a bedding plane or permeability anisotropy of a hydrocarbon bearing formation and using a perforating tool to form in the formation holes having an essentially elliptical or elongated cross-section with the longest axis of said elliptical or elongated cross-section oriented perpendicular to said bedding plane or in the direction of the lowest permeability.
Permeability, which expresses the ease with which fluids flow through rock, varies in general with the direction in which it is measured. This property, often called rock anisotropy, arises at the bedding scale during deposition, grain alignment and packing. The permeability distribution that governs the fluid flow in rocks will result in fluid movement dominated by a horizontal flow.
By creating drain-holes or perforations with greater exposure to the horizontal flow than to the vertical flow, such as elliptical perforations with long axis vertical, a higher productivity can be expected. Although large flow-rates are encountered near the tips in conventional perforations the flow rates through the main body of the perforations are still significant.
Large anisotropies are observed in many formations. Typical measured ratio of horizontal-to-vertical permeability range from 10 to 100 times. Examples are known where lamination of shales in a sandstone matrix results in a ratio of 170 the reason being that shales have small grain size and extremely low permeability. Therefore, the permeability is significantly lower across the sand bed boundaries than within the sand beds. In clean sandstone a permeability anisotropy ratio of 276 has been reported. In this case the permeability anisotropy was caused by variation in grain size and packing. Therefore the perforation of laminated sand-shale formations, in which each layer may have a thickness of 5 cm or less, is a particular concern of the present invention.
Permeability anisotropy can be measured with different methods such as core analysis, well testing or logging techniques (dip meter, micro imagers based on acoustic or resistivity measurements, etc.). More recently a technique for measuring vertical and horizontal permeability uses a multi-probe wireline formation tester in open hole before the completion is run. This technique allows the completion engineers to choose the optimum completion method including a suitable perforation strategy for optimizing productivity.
In a preferred embodiment, the holes are formed using a discharge-type perforation tool. Preferably the perforation tool is creating the perforation in one operational step, i.e., cutting through the casing and the formation without interruption or retrieval of the tool.
Discharge-type perforation tools include perforation guns and jet cutting tools. The latter discharge a fluid jet usually loaded with additional abrasives. With a jet cutting tool, a slit hole is cut into the formation using either guided nozzles, or a suitably arranged set of nozzles or specifically shaped nozzles. Jetting cutting tools as such are known and have been used in the oilfield industry for well cleaning applications, e.g. U.S. Pat. Nos. 4,349.073 and 5,337,819 and the U.K. Patent Application No. GB 2,324,818 A.
However more effective discharge-type perforation tools are likely to be perforation guns using shaped charges as described for example in U.S. Pat. Nos. 5,421,419 and 5,337,819.
For the purpose of the present invention, modifications have to be made as to the design or deployment of such guns. The modification may include using conventional symmetrical shaped charges which produces circular holes but tilting the direction of the perforation hole with regard to the bedding plane. To implement this embodiment of the invention the shaped charges may have to be mounted at an angle of less than ninety degrees for the tool""s main axis. Another modification to conventional gun designs includes the use of charges with no rotational symmetry, specifically shaped charges with an elliptical cross-section as described herein below.
In the earlier U.S. patent application Ser. No. 09/321,040, filed May 27, 1999, incorporated herein by reference, elliptical perforations are described to be stronger than conventional circular perforations thus allowing for greater drawdown pressure and depletion before failure of the perforation due to sand production. The new invention does not contradict the use of elliptical perforations for sand prevention because in most cases, for both sand prevention and maximum productivity, the elliptical perforations have to be oriented in the same way. Hence it is perfectly feasible to achieve an increased production and lower risk of failure using the tools and methods described herein.
In addition, the present invention can be used in strong rock formations where the sanding risk is not a problem, simply for maximizing the productivity.
These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the following detailed description and drawings.